Current commercial anode materials for lithium ion batteries mainly use graphite. However, theoretical specific capacity of graphite is only 372 mAh/g, which can not meet the development requirements of the new generation of high-capacity lithium ion batteries. Silicon has the highest theoretical lithium storage capacity (4200 mAh/g) and a low lithium deintercalation voltage platform (about 0.4V), thus it is the most potential new anode material for lithium ion batteries to replace graphite. However, in the charge-discharge process, silicon exhibits significant volume change, which leads to pulverization of the material particles and the destruction of conductive network within the electrode, limiting its commercial applications. In addition, the intrinsic conductivity of silicon is very low (only 6.7×10−4 Scm−1), thus it is not suitable for high current charge-discharge. On the other hand, the carbon-based material has a small lithium intercalation and deintercalation volume effect and high conductivity. The combination of silicon and carbon can effectively alleviate the volume effect of silicon, reduce the electrochemical polarization, and increase charge-discharge cycling stability. Chinese Patent application CN200510030785.8 discloses a lithium ion battery silicon/carbon/graphite composite anode material, which is prepared by a concentrated sulfuric acid carbonation method. This material consists of elemental silicon, graphite particles and amorphous carbon and does not have a porous structure. Its initial lithium deintercalation capacity is about 1000 mAh/g, but after 10 charge-discharge cycles, the capacity is attenuated by about 20%. Thus, its charge-discharge cycling stability is not good.
To further alleviate the volume effect of silicon, a silicon material having a porous structure is designed. Its internal pore volume reserves space for the volume expansion of silicon, the macroscopic volume change of the lithium storage material is reduced, the mechanical stress is relieved, thus the structural stability of the electrode is improved.
Chinese patent ZL200610028893.6 discloses a copper-silicon-carbon composite material having a nano-porous structure. It is prepared by a high-energy ball milling process. The pore size is 2 to 50 nm, the copper content is about 40 wt %, and the carbon content is about 30 wt %. The material shows a good charge-discharge cycling stability, but its reversible capacity is low, which is only about 580 mAh/g.
PCT/KR2008/006420 discloses a silicon nanowire-carbon composite material having a mesoporous structure. It is produced through an alumina template method. The silicon nanowire has a diameter of 3 to 20 nm, the diameter of the mesopore is 2 to 20 nm, and the carbon content is 5 to 10 wt %. The material has a charge-discharge capacity of 2000 mAh/g at the rate of 1 C. The cycling stability is better, but the process is complex, thus it is difficult to realize industrial production.
Angewandte Chemie International Edition, 2008, Issue 52, pages 10151-10154 reports a three-dimensional macroporous silicon-based material. Firstly, silicon tetrachloride is reduced with sodium naphthalene and butyl lithium is introduced therein to produce butyl-encapsulated silicone gel, followed by the addition of silica particles as a template, and then carbonization is carried out by heat treatment, finally the material is caustic etched by hydrofluoric acid, a macroporous silicon material is thus obtained. The macroporous silicon is of a single crystalline structure, whose average particle diameter is 30 μm or above, and the pore size is 200 nm. The reversible capacity of the material at the rate of 0.2 C is 2820 mAh/g, the cycle performance is good. However, its synthesis process is cumbersome, and a large amount of strongly corrosive and highly dangerous chemical reagents are necessary. The waste would affect the environment, and the production cost is high. Thus, it is not suitable for large-scale industrial applications.
Advanced Materials, 2012, Issue 22, pages 1 to 4 reports a macroporous silicon silver composite material. Firstly, elemental silicon having a three-dimensional macroporous structure is prepared by magnesium thermal reduction, then silver nanoparticles are deposited in the macro pores by silver mirror reaction, the silver content is 8 wt %. The macroporous silicon is of a single crystalline structure, its particle size is 1 to 5 μm, and the pore size is about 200 nm. Its initial lithium deintercalation capacity is 2917 mAh/g, and after 100 cycles, the deintercalation capacity remains above 2000 mAh/g. However, the use of silver would substantially increase the cost of production of the material, which is adverse to its industrial application.